Mechanism of overburden fracture induced earthquakes in coal seam mining

Mining-induced earthquakes are unnatural seismic events that frequently occur in high-position hard and thick rock strata during coal mining. Considering the frequent occurrence of strong mining-induced earthquakes in the Dongtan mining area, this study analysed the fracture migration characteristics of hard and thick rock strata and the focal mechanism of mining-induced earthquakes based on Volasov's thick-plate and moment tensor inversion theories. The results showed that the main key strata were difficult to break under single-panel mining conditions because of the thick and high-strength rock strata and breakage of the main key strata is caused by multiple-panel mining. Volasov's thick-plate theoretical calculation indicated an initial fracture span of the main key strata was 314 m, which is consistent with the actual mining distance of the working face. This verified that strong mining-induced earthquakes were induced by the initial fracture of the main key strata. In coal mining, the pure shear failure type of mining-induced earthquakes indicated the highest percentage, and the shear fracture of rock strata was the primary cause of strong mining-induced earthquakes. The dip angle of the focal fracture surface in mining-induced earthquakes was generally within 15 degrees. Through an analysis of the focal mechanism of mining-induced earthquakes, it has a certain guiding role in explaining the mechanism of mining-induced earthquakes. Based on Volasov's thick-plate theory, the theoretical calculation verified that strong mining-induced earthquakes were induced by the initial fracture of the main key strata. Based on the moment tensor inversion theory, the source rupture types of strong mining-induced earthquakes during mining were inverted. In coal mining, the pure shear failure type of mining-induced earthquakes indicated the highest percentage, and the shear fracture of rock strata was the primary cause of strong mining-induced earthquakes.